Process for production of magnesium

ABSTRACT

Magnesium is produced from a mixture of magnesium oxide and calcium oxide by blending the mixture with silicon or ferrosilicon and shaping the blend in the form of briquets, heating the briquets in an inert atmosphere under temperature and pressure conditions capable of substantially inhibiting formation of magnesium vapor, said temperature being not lower than the melting point of calcium-silicon alloy, and thereby producing calcium-silicon alloy within said briquets and subsequently heating the briquets for thereby reducing magnesium oxide.

Matsushima et al.

[54] PROCESS FOR PRODUCTION OF MAGNESIUM [75] Inventors: TomooMatsushima; Tsutou Odajima, both of Yokohama, Japan [73] Assignee: ShowaDenko Kabushiki Kaisha,

Tokyo, Japan [22] Filed: Dec. 24, 1974 [2]] Appl. No.: 536,l75

[30) Foreign Application Priority Data Dec. 28. [973 Japan 48444648 [52]US. Cl U 75/67 R; 75/3; 75/lO A [5 ll Int. Cl. C22B 45/00 [58] Field ofSearch .7 75/67 R, 67 A, ID A, 3. 75/24 (56] References Cited UNITEDSTATES PATENTS 1390.016 ll/l945 Wagner 75/67 R [451 Nov. 11, 1975 Mageeet al 75/67 R Pons et al. .7 75/67 R Primary Emminer-C. Lovell Ass/sum!Examiner-Mi J. Andrews Attorney Agent, or Firm-Oblon. Fisher. SpivakMcClelland & Maier [57} ABSTRACT 7 Claims, 7 Drawing Figures US. PatentNov. 11, 1975 Sheet 1 014 3,918,959

(min) 5 1O 100 1 B F O 1206C 1100?: F I I fi/ Z 60- .9 G E reaction time(min) 100 F i g. 2

0 Ca 7b $1 atomic fraction U.S. Patent Nov. 11, 1975 Sheet 3 0143,918,959

US. Patent Nov. 11, 1975 Sheet 4 of4 3,918,959

0 2b 40 60 80 10b 12b reaction time( min) Fig.7

PROCESS FOR PRODUCTION OF MAGNESIUM BACKGROUND OF THE INVENTION Thisinvention relates to an improved process for the production of metallicmagnesium by the thermic reduction of magnesium oxide at an elevatedtemperature.

Commercial methods employed for the manufacture of metallic magnesiumare divided under two classes, i.e., electrolytic methods which obtainmagnesium by the electrolysis of magnesium chloride (MgCl and thermicreduction methods, which produce magnesium vapor by thermally reducingmagnesium oxide in a vacuum.

The present invention relates to a vacuum reduction method and, moreparticularly. to the silicothermic reduction method.

These methods reduce calcined natural dolomite or calcined artificialdolomite (mixture of magnesium oxide and calcium oxide) at an elevatedtemperature under a high degree of vacuum by using chiefly silicon orsilicon-iron alloy as the reducing agent and condense the resultantvaporized magnesium to obtain metallic magnesium in a solid or liquidform. These operations are batchwise. The reaction of reduction iseffected by the Pidgeon Process in an externally heated steel-maderetort under conditions of about 1200C and mmHg and by the MagnethermProcess in an internally heated vacuum electric furnace under conditionsof above 1500C and mmHg, for example. The reduction reaction gives birthto a reaction residue, which is discharged in the form of ahigh-temperature solid in the case of the Pidgeon Process and in theform of a high-temperature molten slag in the case of the MagnethermProcess. The capacity for magnesium production per unit reactor, namelyone retort in the Pidgeon Process or one electric furnace in theMagnetherm Process, is 50 to 90 kg/day or 2.5 to 7.5 tons/day. Thus, theunit capacity is much greater by the Magnetherm Process. According tothe Magnetherm Process, the reaction residue is discharged in the formof a high-temperature molten slag and, owing to addition of alumina tothe raw materials, the melting point of the reaction residue is loweredso that at the working temperature of the electric furnace, the reactionresidue retains a mo]- ten state possessed of suitableelectricconductivity. Said reaction residue, therefore, constitutesitself an electric resistor and, because of the Joule effect, functionsas an internal heat source of the electric furnace. Further, the moltenslag can easily be discharged by means of tapping procedure. Because ofthese advantages, this process permits use of a large capacity electricfurnace.

Even with the Magnetherm Process, however, it is difficult to achieveimproved efficiency over the existing level for the reasons to be givenherein below. In the Magnetherm Process, a glanular magnesia-containingsubstance and a granular reducing agent are added intermittently to themolten slag and dissolve therein to cause a reaction of reduction in themolten slag. It is clear from physical chemical theory regarding moltensubstances that the rate of the reaction depends on the activity of M gOin the molten slag. As the reaction proceeds, the reaction velocitydeclines and the yield of magnesium decreases.

An object of this invention is to provide an improved process of notablyhigh yield for the manufacture of 2 magnesium by the dry thermicreduction of magnesium oxide.

SUMMARY OF THE INVENTION To accomplish the object described aboveaccording to the present invention, there is provided a process whichcomprises, as a first stage, blending a substance composed of magnesiumoxide and calcium oxide, generally known as dolomite, with silicon orferrosilicon or a mixture thereof, shaping the blend in the form ofbriquets, subsequently heating the briquets in an inert atmosphere undertemperature and pressure conditions capable of substantially inhibitingformation of magnesium vapor, said temperature being not lower than themelting temperature of calcium-silicon alloy, and thereby formingcalcium-silicon alloy within said briquets, and, as a second stage,placing the briquets containing the calcium-silicon alloy in a steelretort or tightly closed electric furnace and heating the briquetstherein to produce magnesium vapor. The magnesium vapor is solidified ata cooled section of the steel retort or it is introduced into a separatecondenser to be liquefied. The briquets having calcium-silicon alloyformed therein are hard enough to resist disintegration and show highthermal conductivity. Particularly the fact that magnesium oxide and thecalcium-silicon alloy are held in a state of intimate contact within thebriquets brings about highly advantageous conditions for the reductionreaction. Since the reaction velocity is notably high, the rate ofreaction achieved is high and the vapor pressure of magnesium is alsohigh as compared with those obtained by known methods, there is enjoyedan advantage that the operation can be performed not only under apressure above normal pressure but even under a pressure even belownormal pressure, the production rate or yield of magnesium is higherthan that of known methods and production can be made more economical.

The other characteristics and advantages of the present inventionachieved to success from the detailed description to be given hereinbelow with reference to the attached drawings.

BRIEF EXPLANATION OF THE DRAWINGS FIG. 1 is a graph showing the resultsof a test conducted on the rate of reaction in the formation ofcalcium-silicon alloy by the reaction between calcium oxide fromcalcined dolomite and silicon.

FIG. 2 is a graph showing the relation between the atomic fraction of Caand Si and the vapor pressure of magnesium observed in the reduction ofmagnesium oxide with calcium-silicon alloy.

FIG. 3 represents X-ray diffraction patterns substantiating presence ofreactants and products described in the specification of this invention.

FIG. 4 is a reaction apparatus used in Example 1 for calcining crudebriquets.

FIG. 5 is a reducing apparatus used in Example 1 and Comparative Example1.

FIG. 6 is a graph showing the relation between the reaction time and therate of reaction determined in Example 1 and Comparative Example '1.

FIG. 7 is a reducing apparatus used in Example 2 and Comparative Example2.

DETAILED DESCRIPTION OF THE INVENTION This invention relates toimprovements pertaining to and in the method for the manufacture ofmagnesium 3 by the silicothermic reduction of calcined dolomite.

This invention originates in the mechanism of reaction newly brought tolight by the inventors and is concerned with a process for theproduction of magnesium by the thermic reduction of calcined magnesiumoxide with silicon accomplished at an efficiency higher than is obtainedby known methods.

Now, the present invention will be described in detail.

The term calcined dolomite" used herein below in the description shallbe interpreted as either calcined natural or artificial dolomitecontaining magnesium oxide and calcium oxide substantially equimolarilyand products obtained by calcining mixture of magnesium oxide withcalcium oxide and mixtures thereof with carbonates or hydroxides. Theterm inert gas shall refer to gases which refrain from reacting withother coexisting substances in the course of a reaction. Specificallyfor the purpose of the present invention, argon, neon, helium andhydrogen are used either singly or in the form of a mixture as the inertgas.

First, the mechanism of reaction brought to light and forming thefoundation of the present invention will be described with reference tothe results obtained in test.

l. [t has heretofore been held that the solid-phase reduction ofcalcined dolomite by silicon proceeds because the calcined dolomite andsilicon react directly with each other to produce dicalcium silicate, asexpressed by the following reaction formula, and consequently increasethe absolute value of the free energy of reaction which is required forthe reduction of magnesium oxide.

(Solid) (Solid) (Vapor) (Solid) The inventors have demonstrated,however, that in the reaction of calcined dolomite with silicon,magnesium oxide from the calcined dolomite is not reduced directly bysilicon but calcium-silicon alloy is formed first by the reaction ofcalcium oxide with silicon and the reduction reaction between magnesiumoxide and this alloy proceeds subsequently.

The test results covering the reaction velocity in the formation of thecalcium-silicon alloy by the reaction between the calcium oxide from thecalcined dolomite and silicon are shown in FIG. 1. Powdered calciumoxide and silicon were intimately blended in amounts to give a ratio of4 mol of CaO to 5 mol of Si sufficient for producing calcium siliconalloy particularly calcium disilicide (CaSi by reducing calcium oxidefrom the calcined dolomite without excess or deficiency and theresultant blend was shaped in the form of briquets having an apparentdensity of about 1.4 to 2.2. The sample briquets were heated in an inertatmosphere of argon at varying temperatures of 850, 950, 1 100 and l200Cfor a prescribed time. The heating was carried out in a high-frequencyfurnace for the temperature of briquets to be elevated quickly to theindicated levels, so that the initial conditions of reaction and theirpossible effects were negligible. Yields of CaSiwere determined by X-raydiffraction analysis and differential thermal analysis. in the graph ofFIG. I, the rate of reaction in the formation of CaSi is indicated bythe ordinate and the reaction time (in minutes) by the abscissa. Thecoordinates for the temperatures of 1100 and l200C correspond to theupper abscissa graduation and those for the temperatures of 950 and 850Cto the lower abscissa graduation respectively. It is seen from the graphthat the reactions at ll00 and l200C proceed very quickly so that withinonly 5 minutes of reaction, the ratio of formation of CaSi exceeds at l100C and at l200C respectively. In contrast, the reactions at 850 and950C proceed very slowly. A possible reason for the significantdifference in the rate of reaction between the temperature levels of 950and ll0) C may be that at temperatures over about l050C, the formedalloy is in a molten state and the reaction taking place in asolid-liquid phase proceeds at a notably high reaction velocity, whereasat temperatures under 950C, the alloy is in a solid form and does notcontribute to acceleration of the reaction at all.

The present invention resides in applying what the inventors have thusbrought to light to the process for magnesium production. The reactionof the calciumsilicon alloy with the calcined dolomite proceeds in thepaths expressed by the following elementary reaction formulas (1)through (4).

(XCa+ YSi= 7t Ca Si alloy, particularly Casi in these reactions, it doesnot matter whether the substances involved, except the magnesium vaporproduced, are in a molten state or in a solid state. The results broughtto light by the inventors can be explained by the associated reactionformula.

Thus, the vapor pressure of magnesium is represented in accordance withreaction (2) through (4). P denotes the vapor pressure of magnesium, athe activity of silicon and a the activity of calcium respectively.

By using a preparation having magnesium oxide and calcium oxide mixed ata desired ratio, namely a preparation in which the variables m and n inthe aforementioned formulas are selected as desired, as well as by usingdolomite, it is made possible to form a calciumsilicon alloy of thedesired composition and then to accomplish the reduction of magnesium,with the vapor pressure of magnesium controlled as desired.

The variables n and m are reaction fractions of the formulae (2) and (3)respectively. Accordingly, m/n equals the ratio of the correspondingelementary reactions.

2. The conclusion indicated by the present formula has been confirmedthrough tests conducted with a view to formulating a specific processcapable of materializing what the inventors have brought to light. FIG.2 shows the results. Sample briquets having an apparent density of about1.8 were prepared by mixing magnesium oxide, calcium oxide andcalcium-silicon alloy having a different Si/Ca atomic ratio, and shapingthe blends. For the purpose of comparison with the Pidgeon Process inwhich activities of the oxides are in unity, the reaction temperaturewas fixed at 1200C and the heating was given in a stream of argon tomeasure the equilibrium pressure of magnesium vapor. In the graph ofFIG. 2, the ordinate indicates the vapor pressure of magnesium in mmHg(P and the abscissa indicates the atomic fraction of Ca and Si. In thegraph, A represents thermodynamically calculated values and Bexperimentally found values. L is liquidus point of the alloy in whichthe primary precipitation of Ca-Si occurs, L is liquidus point of thealloy in which the primary precipitation of Si occurs. The deviation ofB from A results from the fact that thermodynamic data underconsideration are presumably not settled. ln the curve of B, theobserved value of vapor pressure at the atomic fraction of 0.667Si andof 0.333Ca is found to be approximately equal to the condition expressedby the reaction formula of the Pidgeon Process and this vapor pressureis in agreement with the value observed by investigators. Both theexperimentally and thermodynamically determined curves show that thevapor pressure versus composition depends upon the content of calcium.

3. Then, a study was made of the relation between the reactiontemperature and the vapor pressure of magnesium. Calcined naturaldolomite (containing 31.8% by weight of MgO) and metallic silicon werepulverized to a particle size of finer than 80 mesh, blended in amountsto give a molar ratio MgO/Si=2/l and shaped in the form of briquetshaving an apparent density of about 2.0 g/cm". The sample briquets werepreliminarily heated in the atmosphere of argon under normal pressure atl200C for five minutes to cause formation of calcium-silicon alloywithin the briquets. Then the heated briquets were cooled and crushedinto grains 3 to 5mm in diameter. By the same method as described above,the sample grains thus obtained were tested for equilibrium pressure ofmagnesium vapor. Consequently, there were obtained notably high vaporpressures such as, for example, 70 mmHg at l250C, 180 mmHg at 1350C, 400mmHg at l450C and 1330 mmHg at l600C, clearly indicating a relation oflog P=AIT+B between the logarithmic vapor pressure and the reciprocal ofabsolute temperature, the parameters being A=lO,454 and B=8.706.According to this formula, the reaction temperature required for thevapor pressure of magnesium to reach 760 mmHg in this reaction system iscalculated to be about I520C.

In contrast, the principle of the reaction involved in the operation ofthe Magnetherm Process is expressed by the following formula (5indicating that the reduction of MgO depends on the activity of Sicontained in the silicon-iron alloy used as the reducing agent and onthe activity of MgO present in the multicomponent molten slag.

(in molten slag) (liquid) (vapor) (in molten slag) -12 log m m si MaO550 I wherein, P denotes the vapor pressure of magnesium,

.P denotes the vapor pressure of magnesium of standard statethermodynamically, and a a a denote the activities of Si, MgO and SiOrespectively.

From the standpoint of the melting point of molten slag, however, it isdifficult to maintain reasonably high activity of MgO to improves thevapor pressure of magnesium under consideration. In the neighborhood ofl600C, the vapor pressure of magnesium is at most several tens of mmHgwhen the slag composition is that of normal operation according to theformula (5). According to the present invention the process can beoperated at a notably high vapor pressure of magnesium which isestablished by the reaction under consideration. This advantage bringsabout an effect of inhibiting subsidiary reaction products such as COand SiO which pose a serious problem to the Magnetherm Process.

4. Further, the production of calcium-silicon alloy can be obtained byusing ferrosilicon instead of silicon. It has been demonstrated by X-raydiffraction analysis that in the substitute use, silicon correspondingto the amount of silicon resulting from the deduction of the compoundequivalent to FeSi in the case of 75% ferrosilicon and that from thededuction of the compound equivalent to FeSi in the case of ferrosiliconrespectively take part in the reaction preferentially.

FIG. 3 represents X-ray diffraction patterns obtained by reactants andproducts in the various tests mentioned above. Specifically, thediagrams No. l A through F are standard diffraction lines used to detectreactants and products. The diagram A represents use of pure silicon andthe diagram B use of ferrosilicon; in the range of the used composition,FeSi is also detected. The diagram C represents use of 50% ferrosilicon,although presence of FeSi is detected besides the compound mentionedabove. The diagram D represents a calcium-silicon alloy having acomposition of 62.9% of Si, 30.1% of Ca and the remainder of Fe. Thediagram E represents use of calcium disilicide preparedstoichiometrically. The diagram F represents use of calcined dolomite asthe raw material. The diagrams No. 2 through No. 5 relate to briquetswhich were stoichiometrically prepared, with the aim of producing CaSito include calcium oxide and pure silicon or 75% or 50% ferrosilicon ata molar ratio of said CaO to free silicon of 4 to 5 and which werethereafter subjected to heat treatment in the first stage to produceCaSi and accompanying Ca- SiO so that unreacted Si is recognizable. Thediagrams No. 6 and No. 7 represent the preparations having calcium oxideand silicon blended in amounts to give a molar ratio CaO/Sifl/3 and 4/2respectively, each having silicon content of less than isstoichiometrically required. Substantially the whole of the silicon isseen to have taken part in the reaction, with diffraction lines of CaSiand excess CaO appearing conspicuously. Similar results have beenconfirmed to issue from the reaction between calcined dolomite andsilicon. The diagrams No. 9 through No. 12 represent the briquets havinga molar ratio of CaO/Si=4/5, with pure Si, 90% ferrosilicon, 75%ferrosilicon and 50% ferrosilicon respectively used as the reducingagent. The diagrams furnish a clear proof that silicon does not reactwith magnesium oxide directly but reacts preferentially with calciumoxide to produce CaSi,

The present invention has been accomplished on the basis of the resultsbrought about from the various tests described above. To be morespecific, this invention relates to a process which comprises, as afirst stage, adding silicon or ferrosilicon to calcined dolomite or amagnesium-containing raw material having magnesium de mixed with calciumoxide, homogeneously blendand shaping the mixture in the form ofbriquets, lting the briquets in an inert atmosphere under the iperatureand pressure conditions capable of subntially inhibiting the formationof magnesium vapor 1 thereby giving rise to calcium-silicon alloy withinbriquets and, as a second stage, heating the briquets itaining thecalcium-silicon alloy to effect the rered reduction. In this case, thebriquets are generprepared by pulverizing natural or artificial dolo- 1eand silicon or ferrosilicon to a particle size finer n 80 mesh andblending the resultant powders. For ictical purpose, the raw materialsare desired to be (Ed at a stoichiometric ratio or roughly at a molar ioMgO/CaO/Si=l/l/O.5. In this case, the equimolar io MgO/CaO=l/l ispractically satisfied in natural omite. Where ferrosilicon is used asthe silicon tree, it is adequate to determine the amount of ferrocon soas to meet the aforementioned molar ratio in ms of free siliconequivalent. The briquets thus preed are then heated. As regards thetemperature for s heating, since the ratio of reaction is extremely lowtemperatures under 950 C as indicated in FIG. 1, lower limit of reactiontemperature is fixed at 50C. The upper limit of temperature for thisheating st satisfy the conditions capable of substantially initing theformation of magnesium vapor during the iting. Said conditions areselected from the formula ressing the relation between the temperatureand vapor pressure of magnesium touched upon in (3) the results of testdescribed above. Let T, stand for absolute temperature required forheating the brizts under normal pressure, and the pressure (in iHg) isrequired to exceed the value of P, which is culated from the followingequation.

l0.454 log P, 8.706

r example, the heating is given advantageously at a nperature of about1500C under normal pressure 1 of about l200C under a reduced pressure ofabout mmHg. If the briquets are heated at the temperature isfying theconditions just described, then the reacn leading to the formation ofcalcium-silicon alloy is :elerated, the formed calcium-silicon alloy isobied in a molten state, the briquets consequently ac- .re hardnessenough to withstand disintegration. the rmal conductivity is enriched,the magnesium oxide 1 the reducing agent in the briquets are broughtinto tate of intimate contact and conditions highly advaneous for thereducing reaction in the subsequent p are brought about. The briquetsare subsequently vjected to solid-phase reduction in the following p.If, in this case, they are exposed to a temperature :eeding the meltingpoint of the calcium-silicon alloy itained therein, or even after theyhave thoroughly eased magnesium vapor, the briquets themselves are ainedfast by the magnesium oxide or the high meltdicalcium silicate resultingfrom the reduction and, refore, are kept from being disintegrated. Jow adescription will be given of the reduction to ich the briquetscontaining the calcium-silicon alloy subjected. The reaction conditions(temperature 1 pressure) for this reduction can be selected in acdancewith the vapor pressure of magnesium and reaction velocity.Specifically, the heating is required to be given under a pressure lowerthan the pressure which is calculated from the formula of the relationbetween the pressure P (mmHg) and the absolute temperature T touchedupon in (3) of the test results described above. Let T stand for theabsolute temperature of heating, and the pressure will have to be lowerthan the pressure P (in mmHg) to be calculated from the followingequation.

log P In the operation of the Pidgeon Process or the Magnetherm Process,the equilibrium pressure of magnesium vapor at the operation temperatureis of the order of several tens of mmHg at most. The countertype of thepresent invention plainly exceeds this level. The lower limit of thetemperatures at which the present invention can manifest itscharacteristic features should, therefore, be fixed at l200C (thetemperature at which the equilibrated pressure of magnesium vaporexceeds 40 mmHg). As is evident from (3) of the test results introducedabove, the equilibrium pressure of magnesium vapor notably increases asthe temperature raises from the limit l200C and the degree to which theoperating pressure within the reaction system is decreased can bemitigated in proportion as the temperature increases over this level. Ifthe reduced pressure is maintained, then the reduction is acceleratedand the productivity is improved accordingly. In practical operation,therefore, it is desirable to use as low a pressure as permissibleinsofar as the pressure is not below the equilibrated pressure of COgas. Normally, the reaction can be carried out under a pressure above 25mmHg. Conditions desirable for obtaining high productivity are 5 to 10mmHg at l200C and 50 to 100 mmHg at lSO0C, for example. As concerns theupper limit of temperature for the reduction reaction, no restrictivefactor arises from the reaction itself. It is determined by such factorsas the durability of the reaction system to heat and the thermalstability of the molten slag. With a view to permitting the mineral slagto be discharged in a molten stage, therefore, the upper limit oftemperature determinable from the data on phase diagram of mineral slagis l700C.

When the briquets having the calcium-silicon alloy formed therein areplaced within the heating furnace kept at l200C to l700C, the reductionof magnesium oxide by the calcium-silicon alloy is carried out veryrapidly and a major amount of magnesium vapor is evolved. Then themagnesium vapor is introduced to the condenser and collected in theliquid state. If an internally heated furnace is employed as the heatingfurnace, the molten slag is retained continuously within the furnacethroughout the continuous operation thereof. Depending on the nature ofthe briquets and on the nature of the molten slag, the briquets havingthe calcium-silicon alloy formed therein at times float on the surfaceof the molten slag or may be partially or totally submerged in themolten slag, and complete the magnesium production with notably highvapor pressure of magnesium.

This conclude the description of the present invention. It is furtherpointed out that since the process of this invention can be exercised ata degree of vacuum milder than that involved in the operation of theknown methods such as, for example, the Pidgeon Process and 9 theMagnetherm Process, it enjoys the following advantages.

The fact that the process of this invention can be carried out at a milddegree of vacuum while the other methods require operations to beperformed at such a greatly reduced pressure as l mmHg brings about aneffect of inhibiting the formation of CO gas or SiO gas in the furnaceinterior and of eliminating the degradation of magnesium yield of theaccelerated wear of electrodes due to reoxidation of formed magnesiumvapor. With reference to the reaction between molten slag and theelectrode or lining carbon as experienced by the Magnetherm Process,since the equilibrium pressure of CO at l500 to l600C is more than about17 mmHg, the surface of the carbon electrode or carbon lining within thefurnace is covered with a film of CO gas under such degree of vacuum assaved or mmHg applied in the case of the Magnetherm Process whichresults in the loss of energy, increased electrode load and attendantobstacles and reoxidation of magnesium. The process of this inventiondoes not require the furnace interior to be maintained under a highdegree of vacuum and, therefore, does not entail the various obstaclesjust mentioned.

As is demonstrated by the preferred embodiments of this invention, theprocess of this invention, when carried out by using such furnace as isemployed in practicing the Magnetherm Process, Pidgeon Process or someother similar known process, brings about a notable improvement in theproductivity and yield of magnesium permits continuous feeding of rawmaterials provides effective inhibition of harmful secondary reactions,and so on.

The heat treatment in the first stage and all the treatments in thesecond stage according to the method of the present invention can becarried out in one furnace by using an internally heated furnacecontaining therein a molten slag and having such a construction that thecrude briquet to be cast into the furnace forms therein acalcium-silicon alloy before it reaches the molten slag, and theresultant briquet reaches the molten slag.

Now the present invention will be described more specifically hereinbelow by reference to preferred embodiments. It should be noted,however, that the present invention is not limited to these examples.

EXAMPLE I Calcined natural dolomite (containing 31.8% by weight of MgO)pulverized to a particle size finer than 80 mesh and 75% ferrosilicon(containing 49.5% of free silicon) pulverized to a particle size finerthan 80 mesh were mixed in amounts to give a molar ratio MgO/Si=2/l to2/ 1.1 and the blend was shaped in the form of crude briquets having anapparent specific gravity of about 2.0 g/cm and a maximum diameter rangeof +mm to 40mm.

Then, the crude briquets were placed in a reaction apparatus like theone shown in H6. 4 and heated therein.

In the drawing, 1 denotes an upper hopper, 2 a reaction zone, 3 abriquet receptacle, 4 and 5 a gas inlet and outlet, 6 an externalauxiliary heater, (with a metal or carbon serving as a heating element;a proper external gas flame heater or electric resistance generator maybe used as occasion demands) 7 a high-frequency generator, 8 crudebriquets and 9 briquets having calciumsilicon alloy formed therein. Inthe present example,

argon was introduced as the inert gas through the inlet 4 and dischargedthrough the outlet 5 to effect displacement of the air in the reactorinterior. The heating of the briquets for the formation of saidcalcium-silicon alb y was carried out under a minute pressure (severalmm of water column) of this inert gas. To be specific, when the crudebriquets 8 were fed to the upper hopper 1, they were caused to descenddown the reaction zone 2 (kept at 1050 to l200C), during which descentthey were heated by the high-frequency wave generator 7 and consequentlycaused to produce calcium-silicon alloy therein. The fired briquets 9containing the calci um-silicon alloy were then moved into the briquetreceptacle 3. As is apparent from the diagram, the heating or theformation of calcium-silicon alloy in the briquets could be carried outcontinuously by having crude briquets fed continuously into the upperhopper 1.

When the retention time of the briquets within the reaction zone wasfixed at about 20 minutes, the conversion to calcium-silicon alloyexceeded 95% Subsequently the fired briquets having the calciumsiliconalloy already formed therein were subjected to reduction reaction in areduction apparatus like the one shown in FIG. 5. [n the drawing, 9denotes the fired briquets, 10 the slag, ll a raw material bin, 12 ascreen feeder, 13 an internally heating reaction furnace. 14 a screw, 15a slag discharge outlet and 16 a magnesium vapor outlet.

The fired briquets 9 stored in the raw material bin ll were conveyed bythe screw feeder l2, thrown into the internally heated reaction furnacel3 and retained for a fixed length of time in the furnace interior. Thesolid siag formed was discharged by the screw 14 through the outlet 15.Inside this furnace, the fired briquets were heated and MgO in thebriquets was reduced to give rise to magnesium vapor. The furnaceinterior was maintained under conditions of l0 to 10 mmHg of pressureand l300C of temperature.

Of the results obtained in this example. the relation between thereaction time and the rate of reaction (calculated on the basis of MgOremaining in the solid slag) was as shown in FIG. 6. in the graph, theordinate represents the rate of reaction (72) and the abscissa thereaction time (minutes). The continuous line represents the resultsobtained in this example. The graph indicates that the rate of reactionover 95% was reached very rapidly, namely in a matter of 5 to 10minutes.

COMPARATIVE EXAMPLE l Crude briquets prepared by faithfully followingthe procedure of Example 1 were at once placed in the reducing apparatusof FIG. 5 as used in Example 1 and subjected to reducing reaction. Theresults are shown by the dotted line in the graph of FIG. 6.

As the graph clearly indicates, a reaction time of more than minutes wasrequired for the rate of reaction to exceed The briquets involved in thepresent comparative example exhibited a behavior entirely the same asthat observed in the operation of the Pidgeon Process. Comparisonclearly shows that the process of the present invention is effective innotably improving the productivity and yield of magnesium as evidencedby the results of Example 1.

EXAMPLE 2 The procedure of Example 1 was followed by using entirely thesame raw materials to produce briquets 1 1 having calcium-silicon alloyformed therein. The fired briquets were then subjected to reductionreaction by using an apparatus like the one shown in FIG. 7 under thevarious conditions described hereinbelow.

In the drawing, 2] denotes a vertically movable electrode, 22 acarbonaceous furnace base, 23 a tap for the molten mineral slag, 24, 25and 26 each a briquet bin, 27 the molten mineral slag composedpreponderantly of Ca SiO Al O system, 28 a magnesium vapor pump, 29 amagnesium vapor condenser, 30 a magnesium receptacle and 31 an airdischarge outlet. The apparatus shown in FIG. 7 is an improvedvacuum-tight, single-electrode furnace whose electrode is renderedvertically movable under reduced pressure.

In the present example, the reaction ratio was studied under variousconditions by changing the feed rate of raw materials, with the fumaceinterior maintained under 50 mmHg of pressure and l400 or 1500C oftemperature. The results of the test are shown in the ac companyingtable under the columns, B-l through B-S. In the test, the actual powerload was 48 to 55 KW/kgMg/hr. The reaction ratio was calculated on thebasis of the yield of magnesium.

COMPARATIVE EXAMPLE 2 An apparatus like the one shown in FIG. 7 anddescribed in Example 2 was used, calcined natural dolomite (containing37.4% by weight of MgO and 59.7% by weight of CaO coarsely crushed intograins 5 to 14mm in diameter was thrown into the bin 24, 80%ferrosilicon coarsely crushed into grains 5 to l4mm in diameter was fedinto the bin 25 and powdered alumina prepared by the Bayer Process foruse in electrolysis of aluminum was introduced into the bin 26. Theywere blended to a regulated composition and treated in accordance withthe Magnetherm Process. To be specific, the calcined natural dolomite.ferrosilicon and alumina were blended in amounts to give a weight ratioof 77/14/9 and fed into a molten slag (consisting of 54.8%

12 curred a phenomenon of shelfing around the upper electrode. Noincrease of actual power load resulted in operational improvement.

The results of Example 2 and those of Comparative Example 2 are comparedin the table. This table furnishes a clear proof that the effects of thepresent invention are conspicuous. The data of B series are seen to besuperior to those of A clearly in terms of consumption of raw materials,feed rate of raw materials, yield of magnesium (kg/hr.) and reactionratio of magnesium.

EXAMPLE 3 Briquets having calcium-silicon alloy formed therein wereprepared by faithfully following the procedure of Example 1. Thesebriquets were subjected to operations performed under varying magnitudesof pressure not lower than normal pressure. According to (3) of theresults of test described in the detailed description, the temperatureat which the equilibrium pressure of magnesium vapor issuing from thebriquets of this invention reaches 760 mmHg is calculated to be about1520C. Actually in the present example, however, the reactiontemperature was fixed at I600C. At this reaction temperature, the vaporpressure of magnesium reached 1330 mmHg, a value amply sufficient forsmooth progress of the reaction.

In this example, the vacuum system was so controlled that the innerpressure of the reaction apparatus reached 760 mmHg and 1 I00 mmHgduring the operation. The results of the operation were as shown in thetable under the column, B-6 and B7. The actual electric power againstthe re action was invariably about 5.5 KW/kgMg/hr. As the resultsindicate, the operation of the present invention could be carried outwhen the apparatus interior pressure is higher than normal pressure. Inthis case, both productivity and reaction ratio were decidedly higherthan could be obtained by the conventional process (data under thecolumn A, in the of Cao, 28.5% of SiO 15% of A1 0 and 15-29: of sametable).

Table A B-l B2 B3 B4 B-S as 8-7 Reaction I500 I400 1500 I500 I500 I500I600 I600 temperature (C') Pressure-t mmHg) I0 50 50 50 50 760 I I00Feed rate of raw materiaIs(kg/hrl 625 640 726 869 I I69 I395 I780 I I15Feed rate of raw MgtkgMg/hrl I07 I08 I13 I47 I98 32] 302 I9I Ratio offeed rate of raw material l.U [.02 I.l6 1.39 L87 3.03 2.78 L80 Yield ofMg (kgMg/hr) 92.0 94.2 i I411 I420 I895 294.0 302.0 i820 Reaction ratioof Mg (74] 86.0 37.2 93.2 96.6 95.7 9L6 96.5 95.3

The values for the 8 series are those proportionate to the value (h25ing/hour) for A.

MgO) in the furnace interior at the rate of 625 kg/hour (introducedbatchwise at intervals of 12 minutes) and allowed to react underconditions of 1500C of temperature and I0 mmHg of pressure. The resultswere as shown in the table under the column A. In this case, themagnesium reaction ratio determined by the same method as in Example Iwas 86% and the actual elee tric power against the reaction was 6.5-7.8KW/kgMg/hr. In this case, when the feed rate of raw materials wasincreased, the reaction became remarkably unstable and, owing toconsequent solidification of slag and segregation of raw materials andslag, there oc- What is claimed is:

1. In a process for obtaining metallic magnesium from a mixture ofmagnesium oxide with calcium oxide as the raw material by reducing saidmixture at an elevated temperature to give rise to magnesium vapor andcooling said magnesium vapor, the improvement which comprises twostages, the first stage of adding at least one member selected from thegroup consisting of silicon and ferrosilicon to said mixture consistingof magnesium oxide and calcium oxide, blending and shaping the resultantmixture in the form of briquets, and heating said briquets in an inertatmosphere under temperature and pressure conditions capable ofsubstantially inhibiting the formation of magnesium vapor, saidtemperature being not lower than the melting point of calcium-siliconalloy and thereby giving rise to calciumsilicon alloy within saidbriquets and the second stage of heating, in a heating furnace, thebriquets having calcium-silicon alloy formed therein for therebyreducing the magnesium oxide present in said briquets into metallicmagnesium.

2. The process of claim 1, wherein said heating furnace is an internallyheated furnace containing therein a molten slag and having such aconstruction that the crude briquets to be cast into the furnace areheated to each form therein a calcium-silicon alloy before they reachthe molten slag, and the resultant briquets reach the molten slag so asto allow the reduction of magnesium oxide, in which the heat treatmentin the first stage and all the treatments in the second stage arecarried out.

3. The process of claim 1, wherein the heating in the first stage isgiven at a temperature in the range of from l050 to l500C under apressure exceeding the pressure calculated from the formula:

4. The process of claim 3, wherein the heating in the first stage iscarried out in an atmosphere of argon at a temperature in the range offrom 1050" to l200C for a period between 5 and 20 minutes.

5. The process of claim 1, wherein the briquets having calcium-siliconalloy formed therein are heated at a temperature in the range of froml200 to I700C under a pressure not higher than the pressure calculatedfrom the formula:

log P T 8.706

(wherein, P its the pressure expressed in mmHg and T is said heatingtemperature) in a gaseous atmosphere wherein at least one memberselected from the group consisting of an inert gas and magnesium gas.

6. The process of claim 5, wherein the heating furnace is an internallyheated furnace containing a molten slag, the pressure of the inertatmosphere is at least 25 mmHg, and the briquets having calcium-siliconalloy formed therein are heated in a state such that the briquets floaton said molten slag.

7. The process of claim 5, wherein the heating furnace is an internallyheated furnace containing a molten slag, the pressure of the inertatmosphere is at least 25 mmHg, and the briquets having calcium-siliconalloy formed therein are heated in a state such that the briquets arewholly or partially submerged in said molten slag.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 1 ,95Dated November 11, 1975 Inventor) Tomoo Matsushima et al P 1 f 4 It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 7, should appear as shown on the attached sheets.

Signed and Scaled this twenty sevenlh Day Of April1976 [SEAL] A ttest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer (urmnisslmu'r ujlarvnrsand Trademarks Patent No. 3,918,959 Page 2 of 4 o;-;ido mixed withcalcium oxide, homogeneously blending and shaping the mixture in theform of brizu uets, heating the 'nricpJ-ts in an inert atmosphere underthe temperature and pressure con-.litions ccpablc of substantiallyinhibiting the formation of magnesium vapor and thereby giving rise tocalcium-silicon alloy within the briquots and, as a second stage,heating the briquets containing the calcium-silicon alloy to effect therequired re duction. In this case, the briquets are generally preparedby pulmarizing natural or artificial dolomite and silicon orferrosilicon particle size timer than 80 an? blending the resultantpowders. For practical purpose, the raw materials are desired. to bemixed at a stoichiometric ratio or roughly at a molar: ratio MqO/CaO/Sil/l/C. 5. In this case, the equimolar ratio LlgO/CaO 1/1 is practicallysatisfied in natural dolomite.

Where i'errosilicon is used as the. silicon source, it is adequate tode1;;e.rr:::i2:e the :mount of ferrozailicon so as to meet theaforementioned molar ratio in terms of free silicon equivalent. The

briquets thus prepared are then heated. As regards temper- LLLXJIG forthis heating, since the ratio of reaction is extremely lowtemperatmrz-vs undo: 956C indicated in Fig. l, the lower limit reactiontqsnperature is fixed at 1050C. The upper limit. of. temperature forthis; heating must satisfy the conditions capable of? subsatzmtiall;inixibitinc; the formation of magnesium Patent No 3 ,918 ,959 Page 3 of4 vagor during the ting. Said CUfiditi-Olli; selected. from the.cermui-z expressing the relation be tween the temperature and the vaporpressure 0]. magnesium tcuched 2:70.11 in (3) of the results of testdescribed above. Let T stand for the absolute tempera :re

required far-.1: heating the brique L13 LIA-tier normal prefix-Laue, andthe pressure (in rmfiL is reeuireci to excel-x. the. value 01'. P whichis 1 calculated fZICZP. the folio-wine e-"wvifien.

Patent No. 3 ,918 ,959 Page 4 of 4 For exemple, the heatin is: givena'tw zmtzxgreously at a temperature of about 150 "C under normalpressure and of about l200"C under .7 "viuce-i! prev-zsn'u a o: arm-1t.J'J m5; the Lriquets are heat-e0.

at the temperature satis g'ing the conditions just described, then therz::-1-'"ion lerflin" to the fcnaation of oa1cium-silicon allo AC-Ii.1.: sated the formed celciuua-si3-ic0n LllOY is obt 5.71125 ix.- a

molten state, the briquets consequently acquire hardness GIICZgh towithstand disintegration, the th rmal conductivity is enricherl;

the meqncsium oxide and the r-e'iucinq agent in the briquets areintimate contact and conditions highly brought into a state 0advantageous for the reducing reaction in the subsequent step arebrought about. The briqzzets are subsequently subjected. to solid phase.roouction in the 013. wing step. If, in this case, they are ed to atemperature exceeding the melting point of the c:1l-t:i1in sj.i. co;1alloy cow. ins-3 tr; rein, or e en after they have thoroughl releasedmagnesium vapor, the briquets themselves retained! fast by the magnesiumoxide or the high melting dicalcium silicate resulting from thereduction and, therefore, are kept from being disintegrated.

P3014 .1 description will be given of the reduction to which the hri':ze.:.:-: containing celciuzveilicon alloy are subjected.

The reaction conditions (temperature am. preseure) for this re--cluction can be selected in accordance with the vapor pressure of Cally,the heating mngnerziuta and the reaotion velocity. Specif is re?

1. IN A PROCESS FOR OBTAINING METALLIC MAGNESIUM FROM A MIXTURE OFMAGNESIUM OXIDE WITH CALCIUM OXIDE AS THE RAW MATERIAL BY REDUCING SAIDMIXTURE AT AN ELEVATED TEMPERATURE TO GIVE RISE TO MAGNESIUM VAPOR ANDCOOLING SAID MAGNESIUM VAPOR, THE IMPROVEMENT WHICH COMPRISESK TWOSTAGES, THE FIRST STAGE OF ADDING AT LEAST ONE MEMBER SELECTED FROM THEGROUP CONSISTING OF SILICON AND FERROSILICON TO SAID MIXTURE CONSISTINGOF MAGNESIUM OXIDE AND CALCIUM OXIDE, BLENDING AND SHAPING THE RESULTANTMIXTURE IN THE FORM OF BRIQUETS, AND HEATING SAID BRIQUETS IN AN INERTATMOSPHERE UNDER TEMPERATURE AND PRESSURE CONDITIONS CAPABLE OFSUBSTANTIALLY INHIBITING THE FORMATION OF MAGNESIUM VAPOR, SAIDTEMPERATURE BEING NOT LOWER THAN THE MELTING POINT OF CALCIUM-SILICONALLOY AND THEREBY GIVING RISE TO CALCIUM-SILLICON ALLOY WITHIN SAIDBRIQUETS AND TH SECOND STAGE OF HEATING IN A HEATING FURNACE, THEBRIQUETS HAVING CALCIUM-SILICON ALLOY FORMED THEREON FOR THEREBYREDUCING THE MAGNESIUM OXIDE PRESENT IN SAID BRIQUETS INTO METALICMAGNESIUM,
 2. The process of claim 1, wherein said heating furnace is aninternally heated furnace containing therein a molten slag and havingsuch a construction that the crude briquets to be cast into the furnaceare heated to each form therein a calcium-silicon alloy before theyreach the molten slag, and the resultant briquets reach the molten slagso as to allow the reduction of magnesium oxide, in which the heattreatment in the first stage And all the treatments in the second stageare carried out.
 3. The process of claim 1, wherein the heating in thefirst stage is given at a temperature in the range of from 1050* to1500*C under a pressure exceeding the pressure calculated from theformula:
 4. The process of claim 3, wherein the heating in the firststage is carried out in an atmosphere of argon at a temperature in therange of from 1050* to 1200*C for a period between 5 and 20 minutes. 5.The process of claim 1, wherein the briquets having calcium-siliconalloy formed therein are heated at a temperature in the range of from1200* to 1700*C under a pressure not higher than the pressure calculatedfrom the formula:
 6. The process of claim 5, wherein the heating furnaceis an internally heated furnace containing a molten slag, the pressureof the inert atmosphere is at least 25 mmHg, and the briquets havingcalcium-silicon alloy formed therein are heated in a state such that thebriquets float on said molten slag.
 7. The process of claim 5, whereinthe heating furnace is an internally heated furnace containing a moltenslag, the pressure of the inert atmosphere is at least 25 mmHg, and thebriquets having calcium-silicon alloy formed therein are heated in astate such that the briquets are wholly or partially submerged in saidmolten slag.